This invention relates to high frequency switching circuits and, more particularly, to a high frequency switching circuit using a single-ended switching circuit.
As the power source for computer system peripheral units and conventional communication devices, low-voltage, high-current stabilized power sources have been used. These power sources are, in many cases, switching power sources, which have the advantages of small size, light weight and high frequency. Among these switching power sources, those which make use of voltage or current resonance waves have sufficient switching efficiency of a main switching element (such as a transistor, a thyristor, a MOSFET, etc.) and can operate with low noise. As one variety of the switching circuit used for the prior art switching power source, there is a subclass-E switching circuit. This subclass-E switching circuit is constructed such that a voltage having a waveform corresponding to the arc of the resonance waveform appears between the terminals of a switching element. To this end, the switching cycle and conduction period of the switching element and constants of external circuitry are set to meet predetermined conditions. Of such subclass-E switching circuit, those which use a voltage resonance waveform have power conversion efficiencies in excess of 90% at the operating frequency of several 100 kHz. As another variety of the switching circuit of the aforementioned kind, there is a half-bridge type switching circuit. In this switching circuit, the peripheral circuitry is constructed such that the waveform of current flowing through a switching element corresponds to the resonance waveform. This type of switching circuit has an advantage in that a quenching circuit can be omitted in case when the switching element is a thyristor. In this switching circuit, however, the resonance frequency is determined by the combination of the rating values of construction elements of an external circuit. Therefore, when this switching circuit is used for a stabilized power source or a power amplifier, the control of the output power is complicated. For example, with a forward type switching circuit the power control can be obtained by merely changing the conduction period of the switching element, but with a switching circuit which makes use of a resonance waveform it is necessary to control the switching cycle in a predetermined relation to the conduction period while controlling the conduction period in order to obtain power control while maintaining the resonance waveform. For this reason, the power control is complicated, that is, the control circuit is complicated. Further, although power control may be obtained with a complicated power control circuit, the available range of the control is narrow, and a desired stabilized power source or power amplifier cannot be obtained. Further, with the voltage resonance type switching circuit based on a forward type switching circuit, in which the current flowing through a switching transistor is a triangular current, the utility of the transformer is inferior; for example, where the output is 100 W or above, the bulk of the transformer is greater than that of the transformer of the ordinary forward type switching circuit by 30% or above. Further, since large ripple current flows through a filtering capacitor, a capacitor of a large capacity is necessary.